April 2008 Sky from the Keeble Observatory
How far away can you see the light of a single candle? How about a car headlight?
A single star like the Sun? A galaxy of 100 billion suns? We can work backwards
to get a sense of the answers.
The Andromeda Galaxy is about 2.5 million light years distance, making it the most
distant naked-eye object in the sky. Recalling that the apparent brightness falls
off as the square of distance (e.g. doubling the distance reduces the brightness
by four) we can estimate that the Sun would fade to Andromeda’s brightness if seen
from about 60 light year away. According to a Google search, a single candle can
be seen at a distance of about 3 miles (conveniently, the distance to the true horizon
if your eyes are six feet above the ground). Maine recently passed a law limiting
headlights to no more than 24 candlepower, so they should be visible at 3 miles
times the square root of 24 – about 15 miles.
Why do I raise these questions? Because, on the 19th of March astronomers
detected a titanic explosion from about 7.5 billion light years away – and
its peak brightness made it visible to the naked eye! That’s a lot of candles, equivalent
to 1019 Suns, or 100 million galaxies!
The so-called gamma ray burst was detected by a NASA satellite called Swift. Specifically
designed to detect and monitor gamma ray bursts, Swift continuously monitors about
a sixth of the sky at any one time. If a burst is detected by its Burst Alert Telescope
(BAT) the orbiting observatory automatically slews to point its two other instruments
(an x-ray telescope, and an optical/ultraviolet telescope) at the burst – typically
acquiring the target in less than a minute, and downloading to a network of ground-based
observatories a finder chart so they can also monitor. This particular event is
designated GRB 080319B – the second burst detected on March 19th in 2008.
We understand such bursts as the final collapse of a massive star into a black hole
or neutron star – which will not be the fate of our own Sun. Much of the energy
of the burst forms two counter directed jets which plow through space at nearly
the speed of light. When these jets encounter surrounding interstellar material,
they heat it to extreme temperatures which provide an x-ray and ultraviolet afterglow
which can be further studied to give clues about the originating star, often for
several days or weeks before fading beyond the detection limit of telescopes. This
burst seems to have had one of its jets pointed directly at us. Had it been in our
own Galaxy, it’s unlikely that life on Earth could have survived the intense radiation.
Fortunately, the alignment itself is unlikely (given the infinitely possible other
directions for the jet to point). Also, at 7.5 billion light years, the event also
took place 7.5 billion years ago. Thus far, GRBs have not been detected in nearby
galaxies, and certainly not in our own.
Lunar phases for April: New Moon on the 5th, at 11:55 pm (all
times are EDT); First Quarter on the 12th, at 2:32 pm; Full Moon on the
20th, at 6:25 am; Last Quarter on the 28th, at 10:12 am.
Pre-dawn sky watchers will still see Venus and Mercury rising together before the
Sun at the beginning of April, but they stay very close to the horizon, only reaching
about 6 degrees to the east at sunrise. Venus will still be very bright, so you
may be able to pick it out of the horizon clutter. Jupiter will lead the Sun by
about 3.5 hours; you’ll see it almost due south, about 30 degrees off the horizon
as twilight brightens to full light. Later in the month, Mercury will disappear
into the Sun’s glare, and reemerge about mid-month in the evening skies.
It’s still a good month for evening sky watching, as we have both Mars and Saturn
already up at sunset. Early in April, look for Mars about 80 degrees from the southern
horizon, near Castor and Pollux in Gemini. Saturn is about 2 degrees below and to
the left of Regulus to the east southeast. By month’s end, Saturn will move to about
a degree above Regulus to the south southeast, and Mars will have settled a bit
closer to the southwest horizon at sunset. Mercury will be high above the western
horizon at sunset, and should be visible for nearly an hour before it, too, drops
below the horizon at the end of twilight.
Looking toward zenith, about two hours after sunset at mid-month, we see … not much!
The constellation Leo Minor is in that direction, but it lacks the bright stars
that make it a favorite like Orion. Orion, by the way, is low to the west southwest,
as we bit it adieu until next winter’s skies. Saturn and Regulus are the easiest
markers to find as we look ten degrees further south from zenith.
Sticking with our “two hours after sunset” motif, let’s follow the Moon through
the middle weeks of April. It’s easily found, and will allow us to identify some
of its more prominent neighbors along the ecliptic. On the 7th, we’ll
find the Moon setting to the northwest. On the 8th, we will see it a
bit higher (it sets about 90 minutes later each evening) to the west northwest.
Binoculars will show it passing near the Pleiades cluster. On the 11th
the Moon is now near Mars, passing about a degree below the red planet. The Beehive
Cluster (also known as Praesepe – the Manger – or by its Messier catalog number
M44) will be close to the Moon on the 13th. This cluster is similar in
age and distance to the famous Hyades Cluster, about 577 light years distant and
some 730 million years old. Saturn and Regulus are the next close encounters, as
the Moon will pass them on the 14th and 15th. On the 19th
the Moon passes Spica in the constellation Virgo.
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.